1. Field of the Invention
This invention relates generally to processes for casting optical lenses, and more particularly to processes for casting plastic optical lenses.
2. Description of the Prior Art
Plastic lenses are desirable because they are lighter in weight, higher in impact resistance, and lower in manufacturing cost than comparable inorganic glass lenses. The most common monomer utilized to produce optical plastic lenses is diethylene glycol diallyl carbonate (hereinafter referred to as DAC), which is marketed by AKZO Chemicals Inc. of Dobbs Ferry, N.Y. and PPG Industries, Inc. of Pittsburgh, Pa. under the trademarks Nouryset 200.RTM. and CR-39.RTM., respectively. DAC is widely accepted due to the clarity, strength, impact resistance, resistance to discolorization, and resistance to physical warping or distortion of products made from DAC.
According to current manufacturing processes, the DAC monomer is mixed with free-radical initiator, such as di-isopropyl peroxy dicarbonate. These lens forming materials are injected within a pair of glass mold sections that are sealed by a gasket and held together by a spring clip or other suitable fastener. The charged mold assembly is then heated in a water bath or air oven to a predetermined temperature to cure the resin.
The above described thermal curing process has several drawbacks. As an initial matter, the peroxide catalysts require extreme care in handling. Also, the plastic lenses made from DAC have a refractive index (nD) of about 1.50, which is lower than the refractive index of conventional inorganic glass lenses, the latter being about 1.523. The center thickness and edge thickness of the lenses must, therefore, be made larger than the thickness of inorganic glass lenses.
Efforts have been made to develop compositions which are capable of producing higher refractive index lenses than those currently produced by the polymerization of DAC. Such compounds were found to increase the value of the refractive index, but also required very long curing times, usually 1 to 4 days in length. Also, the resulting lenses typically lack one or more desirable physical properties, such as impact strength, scratch resistance, heat resistance, and colorless transparency.
Conventional thermo-casting techniques require very long curing times, at least about 10 to 20 hours to cast finished (thin) lenses, and about 20 to 60 hours to produce optically acceptable semi-finished (thick) lenses. This low lens productivity requires a large inventory of the expensive glass molds that are used to cast the lenses. There is, therefore, a significant need for a process which will complete the curing in shorter periods of time.
A variety of researchers have developed processes and compositions which shorten the curing time by utilizing ultraviolet (UV) radiation as a driving force to carry out the polymerization reaction. Such processes typically employ a photosetting type resin such as di-functional acrylic monomer and a photoinitiator. The mixture is poured into a transparent mold having a desired optical surface, and thereafter the ultraviolet light is radiated to the resin through the transparent mold to cure the photosetting resin. Examples of such processes are illustrated by U.S. Pat. No. 4,166,088, U.S. Pat. No. 4,298,005, U.S. Pat. No. 4,728,469, U.S. Pat. No. 4,879,318, U.S. Pat. No. 4,919,850, U.S. Pat. No. 5,028,358, and published application WO92/12851. The disclosures of these references are hereby incorporated by reference.
Ultraviolet curing processes are advantageous because of the relatively short curing times, however, problems associated with discoloration, distortion and insufficient hardness are common. The most significant drawback is the striations that are caused by uneven curing and stress. It is known that these striations are the result of thermal gradients in the gel-state, which produce convection lines that become frozen in place and cannot be dispersed. The exothermic nature of the polymerization reaction, and the poor heat transfer of the glass casting mold, are the main cause of the problem. The striations are especially pronounced in thick lenses, where the amount of exothermic heat generated by the polymerization reaction is increased by the increased monomer volume. Efforts to use low temperature, low intensity, and evenly distributed ultraviolet light were largely unsuccessful for casting thick lenses. These processes are, therefore, mostly utilized for the casting of thin, finished prescription lenses.
Lipscomb et al., U.S. Pat. No. 4,879,318, discloses an alternating source of illumination, or the rotation of the reaction cell, to alternate the illumination and to thereby minimize overheating problems. Blum, U.S. Pat. No. 5,028,358, and Blum et al, U.S. Pat. No. 4,419,850, disclose rotation of the mold assembly. Japanese Patent Application Publication No. 3-193313 discloses the use of interrupted illumination to control the polymerization reaction. However, this process does not provide adequate means for determining the cycles of illumination, which will change with changing mold geometry and process characteristics.
It remains desirable to provide a process for the ultraviolet casting of lenses which is faster and more economical than those currently available, and yet which results in the production of high quality lenses which are substantially free of striations and distortions.